Quantum cryptographic key distribution (QKD) systems transmit cryptographic key data encoded in the quantum states of individual optical photons. QKD was first described by C. H. Bennett et al., "Quantum Cryptography: Public key distribution and coin tossing," Proc. Int. Conf. Computer Systems and Signal Processing, pp. 175-179 (Bangalore 1984). The benefits of such a system are that it allows secure transmission of key data over unsecured optical links with security guaranteed by the fundamental quantum properties of light rather than by computational complexity or barriers to interception. This is possible because single photons cannot be split into smaller pieces (intercepted or diverted photons simply won't arrive at the intended destination), nor can they be intercepted and consistently regenerated in identical states since their states cannot be fully characterized by single measurements, leading inevitably to errors in the states of the replacement photons.
Practical systems for distribution of cryptographic keys using quantum cryptography protocols require transmission of single-photon optical signals through some medium, such as optical fiber. Since these protocols encode information in the phase or polarization of the photons, phase and polarization state changes due to mechanical and thermal stresses on the fiber, or to fiber imperfections, must be eliminated or compensated to a degree that permits reliable interferometric detection of the encoded information. In addition, the two parties needing to share a cryptographic key must be able to exchange timing and auxiliary information via a conventional channel.
A technique described by Martinelli in Opt. Comm., 72, 341 (1989) permits automatic, passive compensation for the polarization-transforming effect of the fiber. In this technique light is transmitted through an optical fiber, passes through a Faraday rotator, reflects from a mirror, and returns through the Faraday rotator and fiber. It can be shown that the polarization state of the light returning to the input end of the fiber is always orthogonal to the polarization state of the input light, independent of the polarization transformation induced by the fiber. This effect is referred to as Faraday ortho conjugation.
A quantum cryptographic key distribution system based on a long-path, time-multiplexed interferometry that utilized the Faraday ortho conjugation effect to automatically compensate uncontrolled birefringence effects has been described by Muller et al. in "Plug and play systems for quantum cryptography," Appl. Phys. Lett. 70, 793-5 (1997). The system has excellent interference characteristics (&gt;99% contrast ratio) and clearly shows the value of the auto compensation technique. However, it has several significant weaknesses, including the following:
(1) As implemented, it requires fast phase modulators capable of transmitting both polarizations of light, whereas most available waveguide modulators transmit only a single polarization.
(2) Photons carrying one of the bit values are not sent to a detector. This single detection channel arrangement reduces the data rate by one half.
(3) It requires the use of three Faraday mirrors.
(4) Optical clutter caused by the use of a standard beamsplitter with a do pair of Faraday mirrors to generate a delayed pulse, gives an infinite series of "echo" pulses. Each of these clutter pulses is a factor T.sup.2 smaller than the last (where T is the intensity transmittance of the delay line beamsplitter). The use of a high value of T (T close to 1) means that the echo pulses take many delay line periods to die out, limiting the bit transmission rate.
(5) The single-photon detector sees a reflected laser pulse and an echo series with every shot. In addition, the 2-state protocol used will also direct strong pulses at the detector. These strong extra pulses striking the detector will increase noise counts and limit repetition rate.
The Muller et al. reference also suggests a polarization-encoded system that uses light pulses with specific polarization states, but does not describe an actual implementation.
Thus, what is needed is a practical autocompensated fiber optical system for quantum cryptographic key distribution that eliminates the weaknesses described above.